It is the practice in most commercial and military aircraft to record information in crash survivable modules. A cockpit voice recorder (CVR) maintains a running record of pilot conversation while a digital flight data recorder (DFDR) maintains a record of flight data. These records are useful in analyzing the causes of aircraft accidents. The DFDR is both very expensive and quite heavy, but is not very significant compared to the cost and weight of a large craft.
In light aircraft, on the other hand, the DFDR is unacceptable in both cost and weight, although it is common to use a CVR in small craft, both fixed wing and rotary wing. Still it is desirable to have a crash survivable record of at least some basic flight data in the event of an incident requiring investigation. Preferably a substantial amount of data should be saved since the more information available in an investigation, the higher the chances of an early, accurate assessment of an accident and prevention of a repetition. CVRs do not fulfill this requirement alone and in some cases are of no or limited value. They should be used in conjunction with actual flight data, a combination which has been proven to save lives as well as aircraft.
Thus since the CVR already has a crash survivable module, it is desirable to use that hardware for storing flight data, thereby avoiding the expense of a DFDR as well as the extra weight. A limited capability has already been accomplished by encoding a single parameter data on a carrier frequency superimposed over the flight deck audio. This approach affords very limited flight information and has the potential of interfering with the voice recordings.